Rigid-when-wet paperboard containers and their manufacture

ABSTRACT

Rigid-when-wet, but foldable, corrugated paperboard and process of making same by applying phenolic resin to contiguous surfaces of the outer liner, the medium, and the inner liner without substantially altering the hygroscopicity of the exposed faces of the liners, and adhering the three components together before the resin is cured.

CROSS REFERENCE TO RELATED APPLICATION

This application comprises a division of the application of Robert M.Wilkinson and James R. Lyon, Ser. No. 277,646, filed on Aug. 3, 1972,now Pat. No. 3,886,019 which application comprises acontinuation-in-part of the application of the same inventors havingSer. No. 39,086, filed on May 20, 1970, and now abandoned, prioritybeing claimed with respect to all common subject matter in bothapplications.

BACKGROUND OF THE INVENTION

The invention relates to corrugated paperboard and shipping containersmade therefrom.

In the preparation for market, transportation and storage of perishablecomestibles such as vegetables, fruit and seafood, both the produce andthe container are exposed to much water in one or all of the liquid,solid or vapor states. Such conditions have militated against the use,for that purpose, of containers made of paperboard.

In the marketing of fresh corn, lettuce, celery, peaches, and the like,the prevailing practice is to pack produce into its shipping container,in or near the field in which the produce is picked; and immediatelythereafter to immerse the containers and contents in cold water toremove the field heat from them as quickly as possible. Thereafter, thepackages are stacked, several high, in transportation vehicles or incold storage chambers. The transportation vehicles are frequentlyrefrigerated by ice, frequently in the form of flakes, charged into thebody of the vehicle over and around the stacked packages of produce. Icemelts. In the course of the journey, depending upon its length andweather conditions, the shipment may be re-iced one or more times.Apples present a different aspect of the same problem. Apples areharvested in the autumn, but consumers expect to be able to buy them inother seasons. Apples are cold stored under high humidity conditions,e.g., 90% relative humidity, in boxes stacked ten to twenty feet highfor months.

Ordinary corrugated paperboard containers lose their stiffness when wet.They collapse under the conditions described in the last paragraph.Damage to the packaged produce results. The higher the containers arestacked upon each other, the greater the load on the bottom one of thestack, and the greater the likelihood of collapse in the lower ones withresultant tendency of the stack to topple.

The problem is not a new one. It was referred to in U.S. Pat. No.1,592,824 as follows:

"The use of boxes made of corrugated straw board has, up to the presenttime, been limited to uses where boxes and their contents are notsubjected to moisture and becomes inefficient when wet or damp. Forexample, corrugated board boxes containing goods have not been kept incold storage warehouses, because the moisture would cause the corrugatedboard to disintegrate under pressure of stresses and thus destroy theefficiency of the box as a container. A desideratum in this art has beento provide a corrugated board container which was made of facedcorrugated straw board in usual manner and which would not be renderedincapable of performing its function when subjected to moisture."

"The primary object of the present invention is to produce an improvedcorrugated board which has been treated, after the board has beenmanufactured, with a water-proofing compound, so that it will not absorbmoisture." (Col. 1, LL. 10-34)

It was further discussed in an article by D. J. Fahey entitled "Use OfChemical Compounds To Improve The Stiffness Of Container Board At HighMoisture Conditions," which appeared in Tappi, issue of September 1962,where data are tabulated concerning the physical properties of"Wet-Strength" paperboard sheets, of the kind used as components("liner" and "medium") of corrugated container-board, to which variousresins had been added at different stages (pulp slurry, size press, orsmoothing press) of the paper-making process. Fahey concluded:

"Certain chemicals can be used effectively to improve the compressivestrength of paperboard at high humidities, with the phenolic resinsbeing one of the more promising. The improvement is dependent upon theamount of the resin present in the board, the nature of the resin, andthe way in which it is distributed in the board. Embrittlement of theboard is one of the results of the treatment, but this is minimized withsmall levels of treatment. For the most efficient use of a specificamount of resin, surface treatments are more desirable than thoroughimpregnation of the board. Results point toward a skin effect on theboard which may be achieved with treating mixtures of higher viscosity.Many chemicals that are not too effective at high moisture are highlyeffective at low moistures.

"While the work to date shows some measure of success, further researchis needed to achieve a board that will maintain its stiffness over theentire moisture range."

Comparable date on similarly treated paperboard are reported in U.S.Forest Service Research Note PPL-084, December 1964, with the followingconclusion:

"Compressive strength of linerboard exposed to high moisture conditionscan be improved by treatments with water-soluble phenolic resins. Thelow-molecular-weight type resins gave the higher compressive strength,but these also cause more embrittlement of the board than themedium-condensed phenolic resins."

Another article entitled "Phenolic Resin Treatment Improves FibreboardCompressive Strength" appeared in the October 1965, issue of PackageEngineering, and reported:

"The use of phenolic resins for improving the wet compressive strengthof paper is not new. However, one reason it has not been widely used incontainers is that paperboard containing polymerized phenolic resinoften becomes brittle. Obviously, a brittle corrugated fibreboard isdifficult to score or fold without seriously fracturing the material atthe score lines."

It is the primary object of the present invention to provide corrugatedpaperboard, containers made therefrom, and a process of making the same,which, while providing rigid-when-wet strength sufficient to withstandthe rigorous conditions of flooding with water, storage under highhumidity, etc., while laden as related above, are not objectionablybrittle.

SUMMARY OF THE INVENTION

The present invention is predicated upon the discovery that the teachingof the aforesaid patent was wrong in three respects, to wit: that thecorrugated board be treated after it "has been manufactured," that it betreated so that "the outer faces -- will not readily absorb moisture",and that the inner faces be "treated with an efficient water-proofingmaterial"; and that the efforts to accomplish the desiderata in thecourse of the paper mill operations producing "Wet-Strength" paperboard,as reported in the three publications above mentioned, overlooked thatthe subsequent reeling of the paper had an effect of the same kind as,albeit lesser in degree than, corrugating or folding it.

Accordingly, the present invention contemplates surface-treating thatface or faces of the corrugated container-board components (outsideliner, medium, and inside liner) which will not be an exposed face(either on the outside or on the inside) of containers made from thecomposite container-board, but leaving such exposed faces in suchcondition that they will readily absorb moisture. The surface-treatingoperation can be, and preferably is, carried out concurrently with thecorrugating and laminating operation, but can be carried out in advanceof the corrugating and lamination operation if appropriate precautionsare taken to prevent or minimize the occurence, as in thechemically-active surface-treated materials, of substantial change ofstate while the treated paperboard components await conversion. Thesurface treatment involves the application of a carefully controlledamount of a curable thermosetting resinous material in a liquid vehicle,such as water, which is removed within a matter of seconds withoutsubstantially advancing the cure of the resin. As applied, the solutionor emulsion preferably has a fluidity (e.g., vicosity of 18 ± 0.5seconds in a #2 Zahn cup at 88° F.) approaching that of water (16seconds), will coat and partially penetrate the surface-treatedpaperboard component, and, to a limited extent, migrate therewithin; butthe invention contemplates that such penetration and migration bearrested before the resin has reached the surface of the board which wasnot surface-treated. By comparison, the resinous solution having aviscosity of 17.5 seconds in the Zahn cup approximately a 25 percentsolution solution by weight, while a 18.5 seconds in the said Zahn cuptest approaches a 65 percent solution. The arrest of penetration of thesolution is readily achieved by evaporating the vehicle within a matterof seconds after the resinous mixture is deposited upon the surface ofthe rspective components. The resinous material is peferably a phenolicresin of a type which is not hydrophobic, but cures stiff and waterinsoluble. Depending upon strength and bruise-resistance of the produceto be shipped, one or more of the components can be made of high"wet-strength" paperboard, but for rigid, bruise-resistant produce likewatermelons, and some squash, such is not essential. The severalsurface-treated components are laminated, and adhered together at theirsurface-treated faces, with an adhesive which is compatible with, andmaintains a durable tack with, the resinous material in the presence ofwater, heat and/or cold. Resorcinol-starch compositions are thepresently preferred adhesives. After the surface-treated components havebeen adhered together, the composite corrugated container-board may becut, scored and slotted to form blanks for boxes, and the"manufacturer's joint" completed, but as some stage, before the box ispacked, the resinous material must be cured, as by exposing it tosuperambient heat for the requisite period of time, which variesinversely with the curing temperature.

Surface treating of the components of paperboard in the manner asdescribed in this invention furnishes a container-board, the componentsof which have only a thin film or thickness of curable resin applied tocertain surfaces, with very little impregnation, so that rigidity isprovided in a container-board which, at the same time, still has itsexposed surfaces readily absorptive of moisture, and exhibiting theflexibility normally provided in such board. Therefore, the blanks orboxes which are formed from the container-board which has been treatedin the manner of this invention can still be used and folded in thenormal use of such containers, but at the same time, have enhancedrigidity under moisture exposure conditions. But, since in the conceptof this invention only discrete surfaces of the components of thecontainer-board have been treated, the containers do not exhibit thatfriability that exists in other paperboard which has been impregnated orsaturated with a resinous composition, and which have a tendency tofracture when exposed to weight, impact, or pressure of any sort.

The same principles are applicable to the manufacture of rigid-when-wetsolid fiber container-board.

The mode of applying the resinous material to the paperboard componentsis important not only from the standpoint of uniformity and quantum, butalso from the standpoint of ultimate performance in use, although thereason for the latter has not yet been technologically explained.Superior ultimate performance has been achieved by printing the resinousmaterial onto the surface or surfaces of the respective components, asby a technique known in the printing art as "offset gravure," whereinthe fluid material is picked up from a supply by a metallic roll whosesurface is engraved or milled to provide miniscule cavities or "cells"which entrap the resinous solution, and after the excess is scraped offthe surface, the accurately metered cell contents are transferred to arubber roll (which is more readily wettable by the resinous materialthan is the metallic roll), and therefrom to the paperboard (which hasgreater affinity for the resinous solution than does the rubber roll).Metallic gravure rolls with miniscule cavities or cells engraved ormilled into the outer surface thereof are commercially available withnumerous sizes, and three different shapes, of cells. The shapes andtheir respective utilities were epitomized in an article entitled "VMASeminar Studies Coating Rolls" which appeared in the February 1970,issue of Paper Film & Foil Converter, as follows:

"Roll suppliers generally offer a choice of three different cell shapes:the quadragravure, the pyramid, and the tri-helicoid. As noted, the quadis primarily designed for gravure type of coatings, with viscosities abit heavier than water. The pyramid is for very aqueous formulations,since the sharp point of the upside-down pyramid retards release of thecoating material from the cell. The tri-helicoid is primarily for highlyviscous coatings like adhesives and asphalts."

contrary to those criteria, we have discovered that, despite the lowviscosity of the aforesaid surface-treating mixture, a superior endproduct results when a "tri-helicoid" roll is used to apply it, with the"quad" and the "pyramid" next in that order; but we are as yet unable totheoretically explain the phenomenon. In the "tri-helicoid" roll, thecells are miniscule V-shaped grooves formed helically on the surface ofthe roll so that their length is many times greater than their width ordepth. The "quad" and the "pyramid" each have cells which aresubstantially the same dimension in width as in length, but the"pyramid" is deeper than the "quad" by the degree that the latter isblunted by truncation. Results heretofore achieved indicate that a"tri-helicoid" roll having 54 cells per square inch is to be preferredfor surface treating liner board with phenolic resin compositions havinga concentration of 50 ± 3% of chemically-active ingredients, and the lowviscosity aforesaid, at the rate of 3.6 ± 0.3 pounds of cured phenolicsolids per thousand square feet applied on one face only. On the otherhand, for the application of the same surface-treating composition tothe medium (both sides) at the rate of 1.7 ± 0.2 pounds of curedphenolic solids per thousand square feet per side, it is preferred thata tri-helicoid roll having 95 cells per square inch be used, but it willbe understood that with other concentrations of chemically-activeingredients, rolls with etching of different size or type may be usedaccording to the criterion that the higher the viscosity, the larger thecell.

The curable thermosetting resinous materials utilized in this invention,of which there are a variety readily available upon the market asdescribed in this application, and any catalyst that may be used inconjunction with the thermosetting phenolic materials, are mosteffectively and practically entrained in concentration in a vaporizableliquid carrier within the range of between 35 to 65 percent by weight.At these concentrations, the application of the mixture to thecomponents of paperboard as through surface treating provides fordeposition and retention of more resinous material proximate the treatedsurface, and due to the higher concentration of the resin solution,impregnation of the board is significantly reduced.

Within limits, the higher the "wet-strength" of the paperboardcomponents as produced at the mill, the greater will be therigid-when-wet strength of composite container-board, and of containersmade from it, whose components were surface-treated in accordance withthe invention. However, the "wet-strength" of the components beforesurface treatment should not be so great that the medium is fractured inthe corrugating operation, or that the liners are fractured by the bendsto which they are subjected during surface treatment and/or adhering themedium to them as is done in the conventional mode of manufacture ofdouble-faced corrugated container-board.

Typically, the liners and the corrugating medium, which are to besurface-treated and become components of the ultimate rigid-when-wetcorrugated container-board, may have the characteristics enumerated inthe following table:

                  TABLE I                                                         ______________________________________                                             Caliper  Weight                                                          Type (inches) (pounds per 1,000 sq. ft.)                                                                       Wet Strength                                 ______________________________________                                        LINER BOARD                                                                   1    0.016-8  62                 Yes                                          2    0.018-20 69                 No                                           3    0.012-4  42                 No                                           4    0.018-20 69                 Yes                                          CORRUGATING MEDIUM                                                            5    0.011-2  36*                Yes                                          6    0.008-9  26*                No                                           7    0.011-2  36*                No                                           8    0.010-1  33*                Yes                                          ______________________________________                                         *To arrive at weight of medium in 1,000 sq. ft. of the composite              corrugated container-board, add the appropriate industry standard             corrugation take-up based on the flute used, e.g., 54% for A Flute, 33%       for B Flute, and 44% for C Flute.                                        

Among the thermosetting phenolic materials which have produced thedesired results when applied as hereinbefore described are:

I. One identified herein as "Phenolic X" obtainable from MonsantoCompany, the precise chemical composition of which is not now known, butwhich analysis shows to consist of 25.8% by weight of phenols and 22-27%by weight of formaldehydes, the balance (47 to 53% by weight) water. Tomake the solution which is applied to the several paperboard components,"Phenolic X" is mixed with water and a catalyst solution identifiedherein as "Catalyst X," also obtainable from Monsanto Company, theprecise chemical composition of which is not known, but which analysisshows to consist of 156.5 grams per liter of ammonium chloride, 400.4grams per liter of ureas, balance (to make 1000 grams) water andunknowns; and having a pH value of 6.2. The components are preferablymixed in the proportions of 100 pounds of "Phenolic X" to 15 pounds of"Catalyst X" plus whatever additional water is required to obtain aviscosity of about 18 seconds in a #2 Zahn cup at 88° F.

II. One disclosed in Example 2 of U.S. Pat. No. 2,245,245 which is cutwith water q.s. to make an emulsion having a viscosity of about 18seconds in a #2 Zahn cup at 88° F.

III. One disclosed in Part A of Example 1 of U.S. Pat. No. 3,161,547which is cut with water q.s. to the desired viscosity as recited in Iand II above.

IV. One known as "Tybond 990" obtainable from Pacific Resins &Chemicals, Inc., which is represented as a phenol-formaldehyde-watersolution containing 65% solids, and which is cut with water q.s. toachieve the viscosity stated in I and II above.

Other examples of resinous compositions that may be utilized effectivelyin this invention include the phenol-aldehyde resole resin system ascombined with a polyvinyl acetate as identified in U.S. Pat. No.3,607,598; the phenol-aldehyde resin system as combined with aninorganic ammonium salt and urea as described in U.S. Pat. No.3,616,163; the aninoplast modified phenol-aldehyde resole resincomposition as set forth in U.S. Pat. No. 3,617,427; the resin systemdescribed in U.S. Pat. No. 3,617,428; the mixed resin system identifiedin U.S. Pat. No. 3,617,429; the modified phenol-aldehyde resin systemdescribed in U.S. Pat. No. 3,619,341; the resin system described in U.S.Pat. No. 3,619,342; the resin system described in the U.S. Pat. No.3,697,365 entitled Rigid-When-Wet Boxboard, by Abraham J. Reisman andThomas B. Wilkinson; the resin system described in the U.S. Pat. No.3,687,767, entitled Scoring Process for Certain Rigid-When-WetCorrugated Fiber Board, by Abraham J. Reisman and Thomas B. Wilkinson;and the resin system described in the U.S. Pat. No. 3,682,762, entitledRigid-When-Wet Boxboard, by John R. LeBlanc; all of the inventions,their U.S. patents or applications, and the invention of thisapplication being owned, jointly, by a common assignee.

The adhesive used to secure the surface-treated faces of the linercomponents to the crowns of the corrugated medium may be any one whichmaintains tack with the phenolic-treated liners and medium, while wet ordry, hot or cold, and especially must maintain tack under thetemperature at which the phenolic resin is cured. Any of a variety ofaromatic-alcohol-containing adhesives, especially those whichmolecularly bond with the phenol under curing conditions, may be used,but most consistent results to date have been achieved with an adhesivemixed at the site of use from constituents obtainable from A. E. StaleyManufacturing Company, and identified, respectively, as "STAY-BIND"5035, 2100 and 77. 5035 comprises 29% resorcinol and 71% cornstarchhaving an amylose content of abourt 55%; 2100 consists of cornstarchhaving an amylose content of about 55%; and 77 is a "thick boiling"modified cornstarch. The procedure for manufacturing a 650 gallon batchof the adhesive involves: charge a first mixer with 100 gallons of tapwater at ambient temperature; add 300 pounds of 5035 and 120 pounds of77, then agitate until smooth, after which add 30 pounds of caustic sodadissolved in 5 gallons of water, and thereafter heat the mixture to 160°F., maintaining that temperature for 10 minutes. Then dilute with 60gallons of additional water, and agitate for 5 more minutes. In aseparate mixer containing 300 gallons of water at 90° F., add, while thewater is being agitated, 280 pounds of 77, 1,000 pounds of 2100, and 128pounds of formalin (37% formaldehyde solution). Thereafter, slowly addthe contents of the first mixer to the second mixer over a period of tento fifteen minutes, and continue the agitation for one hour, includingthe time for adding the contents of the first mixer to the second mixer.If a viscosity increase is noted by the end of that hour or thereafter,add tap water in one continuous operation until the batch has reached650 gallons, while maintaining the temperature between 105° and 110° F.

After having surface-treated one face (i.e., that one which will beconcealed in the end product) of each of the two liner webs and bothfaces of the corrugating medium web, the respective components aredehydrated to the extent of removing from them that amount of waterwhich was introduced with the resinous composition, care being takenthat in the removal of such water, the deposited thermosetting resindoes not have its cure substantially advanced. After the removal of suchwater from the respective webs, the medium can be immediately corrugatedand adhered to the respective liners in the usual way of makingcorrugated container-board. Therafter, the corrugated container-boardcan be cut into blanks and stored indefinitely under conditions which donot substantially advance the cure of the thermosetting resin) untilconvenience permits them to be subjected to an appropriate treatment forcuring the resin in and on their components; and thereafter quenchingthe resin, as by spraying them with, or immersing them in, cold water.

For maximum efficiency, the surface treatment of the liners and mediummay be carried out as an adjunct to the operation of a conventionalmachine for the manufacture of corrugated container-board, but it willbe understood that, if and when desired, the several components may beseparately surface-treated and stored, under conditions which do notsubstantially advance the cure of the deposited resin, until it isconvenient to run them through a corrugating apparatus in the usual way.

In the accompanying drawings:

FIG. 1 is, at once, a schematic view of an apparatus suitable for, and aflow sheet illustrating the sequence of steps in, the manufacture of therigid-when-wet corrugated container-board as an adjunct to aconventional corrugating machine;

FIG. 2 is a plan view of a double-slotted container blank of a typewhich may be cut off, slotted, and scored, in the concluding operationof the apparatus shown schematically in FIG. 1;

FIG. 3 is a perspective view of the blank shown in FIG. 2 in flat foldedor knocked-down condition, after its manufacturer's joint has beencompleted;

FIG. 4 is a perspective view of a shipping container which results fromsetting up and closing the bottom flaps of the flat folded blank shownin FIG. 3;

FIG. 5 is a schematic view of apparatus appropriate for curing thethermosetting resin deposited on and in the components of containerblanks of the character shown in FIGS. 2 or 3, and thereafter quenchingthem;

FIG. 6 is a perspective view of a portion of gravure roll of the"tri-helicoid" type;

FIG. 7 is a sectional view taken along line 7--7 of FIG. 6 on enlarged(approximately 50 times) scale;

FIG. 8 is a set of graphs illustrating the degree and longevity ofstrength retention under water with boxes made in accordance with thepresent invention in comparison with those of the prior art;

FIG. 9 discloses a micrograph of a medium surface-treated on one sidewith the resinous material;

FIG. 10 discloses a micrograph of liner-board surface-treated on oneside with the resinous material; and

FIG. ll discloses an enlargement view of that portion of the micrographin FIG. 10 indicated by the longer-tailed arrow.

Referring now to FIG. 1 for an illustrative embodiment of the apparatusfor carrying out the process of the invention as an adjunct to aconventional corrugating machine, there is provided a plurality of resinapplicators 1, 2, 3 and 4, which are disposed, respectively, forapplication to the reverse face of a liner web 7. The reverse face isusually the wire-side of the paper, but, in any event, is that face of aliner which will be concealed in the finished product. The applicators1, 2, 3 and 4 preferably include a gravure roll of the type known in theart as "tri-helicoid" and shown in FIG. 6, or the type known as "quad."However, the cells in the gravure rolls for applicators 2 and 3 arepreferably of magnitude such as to transfer to any one side of themedium web only about half the quantity of resin as that applied to asingle side of the respective liner webs by applicators 1 and 4.

Immediately adjacent the respective applicators, there is provided aheater roll, or series thereof, 8 for the web 5, 9 for the web 6, and 10for the web 7. The temperature of the respective heaters 8, 9 and 10 iscontrolled in coordination with the duration of surface contact of theseveral webs with their respective heaters, so as to: (a) avoidsubstantial curing of the applied thermosetting resin; and (b) remove atleast that amount of water which was introduced with the resin solution,and as much other paper-entrained water as is desirable for bestoperation of the particular corrugating machine, which is usually in therange of 7 to 9% by weight of the web prior to being surface-treated. Ineffect, the heaters 8, 9 and 10 are controlled to remove from the webs5, 6 and 7, respectively, that amount of water which was added to theweb by the immediately preceding resin applicator or applicators, as thecase may be. Typically, with a resinous composition of the typedescribed in I above, the temperature of the heaters 8, 9 and 10 may bemaintained at 350° F. with contact duration of 0.5-2.0 seconds. Theduration of the contact may be shortened by increasing the speed atwhich the web moves or by decreasing the arcuate length of the "wrap" ofthe web about a heater roll, or by a combination of the two variables.Conversely, the duration of contact may be decreased by decreasing thespeed and/or increasing the "wrap."

Between heater 8 and heater 9, there is provided the conventionalapparatus 11 for corrugating the medium web 6, as well as an applicator12 for applying adhesive to the exposed crowns of the just-formedcorrugations in web 6 in accordance with the usual practice in themanufacture of corrugated paper-board, where a backup roller, such as13, is employed for pressing the liner web 5, (commonly called the"single face liner") against the exposed crowns of corrugated medium web6 before the latter has departed engagement with one of the corrugatingrolls 11.

Another adhesive applicator 14 is provided for applying adhesive tothose crowns of the corrugated medium web 6 which were addressed awayfrom applicator 12 when the medium traversed it.

As is conventional with corrugating means of the type schematicallyillustrated, the liner web 7 (commonly called the "double-backer"),after leaving the heater 10, is directed in converging relationship withthe previously united single face liner 5 and corrugated medium web 6 ata position immediately beyond the adhesive applicator 14. In theapparatus schematically illustrated in the drawing, such convergenceoccurs on table 15, which may, if desired, be equipped with any suitablebackup means for biasing the united liner 5 and medium web 6 toward theliner 7 with force less than sufficient to collapse the corrugations inmedium 6. At some suitable location on table 15 or elsewhere, thecomposite container-board, consisting of the united components 5, 6 and7, is operated upon by appropriate apparatus for cutting off, scoring,slotting, and/or slitting, the continuous composite web to producecontainer blanks 16, such as that shown in FIG. 2, which is for aso-called "Regular Slotted Container," wherein the blank consists of endpanels 21 and 23 with side panel 22 intervening and side panel 24therebeyond having a so-called "manufacturer's joint" flap 25. Each ofthe panels 21-24 has an adjoining top and bottom flap delineated bylongitudinal score lines 26 and 27 respectively, and by the interveningtransverse slots 28 and 29 respectively.

Blanks such as 16 may be accumulated and stored under ambient conditionsindefinitely, or they may be immediately converted into "flat folded" or"knocked-down" containers by completing the manufacturer's joint whichinvolves securing, as by adhesive or stitching, the manufacturer's jointflap to the remote edge of end panel 21, so as to produce the flatfolded structure shown in FIG. 3, which is the knocked-down container,or the flat folded structure may be folded into a shipping container, asshown in FIG. 4.

Either before or after the manufacturer's joint is completed as justdescribed (or, at any rate, prior to the time the container is packed),the blank or the knocked-down container is subjected to any appropriateoperation for curing the resin with which the components of thecorrugated board were surface-treated. An appropriate curing facility isillustrated in FIG. 5, which diagrammatically illustrates a tunnel ovencomprising an endless conveyor 30, having a multiplicity of wicketlikeflights 31 projecting outwardly therefrom. The endless conveyor isdriven in the direction shown by the arrows in FIG. 5. An accumulationof blanks such as 16, or, alternatively, an accumulation of theknocked-down containers as shown in FIG. 3, is moved into convenientproximity with the input end 32 of the conveyor 30, and one by one therespective blanks are placed upon a passing flight 31 and transportedthereon through the tunnel 33 to the discharge end 34 of the conveyor.While the blanks are being transported on the conveyor, the tunnelenclosing the latter is supplied with a continuous draft of hot airthrough duct 35. The temperature of the air is so coordinated with thespeed of the conveyor 30 that the respective blanks (or knocked-downcontainers) being transported by the conveyor are elevated to thetemperature sufficient to cure the resin previously applied to theinside surfaces of the liners 5 and 7, and to both surfaces of themedium web 6, and which has been reposing thereon and therein in uncuredcondition. An appropriate curing temperature for the phenolic resinshereinbefore described is 375° F. for a period of eight minutes, buthigher or lower temperatures may be employed with appropriate adjustmentof the duration of the exposure of the blanks to that temperature, inaccordance with the empirical rule that for every 18° F. of temperatureabove (or below) the median of 375° F., the duration of the treatment isdecreased (or increased) by 50 percent. Preferably, the blanks orknocked-down containers undergoing curing are maintained at the curingtemperature for the requisite period of time during the middle oftwo-fourths of their movement on conveyor 30, so that in the finalfourth of such movement, they may, if desired, be exposed to a draft ofcooling air supplied through duct 36 which reduces their temperature to,at most, about 210° F. by the time they reach the discharge end 34 ofconveyor 30. The drafts of hot and cold air are preferably separated byan air curtain emerging from an elongated nozzle 37.

From the discharge end of conveyor 30, the blanks and/or knocked-downcontainers are deposited upon a reticulated conveyor 38 which transportsthem, in spaced relationship with each other, under a spray of wateremerging from spray heads 39 above the conveyor, and over a spray ofwater forcibly emerging upwardly from spray heads 40 located below theupper reach of the conveyor 38. At the discharge end of conveyor 38, thewet blanks 16 are stacked and aged for a period of at least four hoursunder ambient conditions. Thereafter, if the manufacturer's joint hasnot theretofore been completed, the same may be done at any convenienttime and place. Either the blanks or the knocked-down containers arethen ready for shipment to the packer, where they are set up, packed andthereafter subjected to hydro-cooling, or other appropriaterefrigeration prior to, and/or during, shipment.

To demonstrate the nature and extent of the phenolic's distributionwithin the substance of liner-board surface-treated in accordance withthe invention on one face only and cured, versus that of medium-boardsurface-treated on both sides, and cured, scanning electron-micrographsof thickness cross-sections of each, are informative. FIG. 9 shows athickness cross-section of medium surface-treated on both sides with theresin-catalyst-water composition described under I hereinbefore, andcured without being corrugated or combined with liner. It shows thatsurface treated on both sides results in three significant factors: (i)that the resin (the highlights) has penetrated all the way through thethickness dimension of the medium; (ii) that there is less concentrationof the resin adjacent the surfaces than in the intermediate zone; and(iii) that the resin perimetrically encases the bundles of cellulosefibers, but does not fill the familiar canal within them. Note theannuli with central black spots, some of which are indicated by arrows.Such an annulus is a bundle of cellulose fibers, the highlighted ringaround it is a sheath of resin, and the black spots are the canalswithin them which are substantially devoid of resin.

In contrast with (i) and (ii) of the micrograph shown in FIG. 9, themicrograph in FIG. 10 shows a thickness cross-section of liner-boardsurface-treated, on one side only with an amount of resin equal to thesum of that applied to each side of the medium-board and cured, itreveals three significant factors: (iv) that the greatest concentrationof resin (highlights) is adjacent the surface at which it was applied;(v) that a substantial increment of the thickness adjacent the surfaceopposite that to which resin was applied is almost completely devoid ofresin; and (vi) like the micrograph of FIG. 9, that the resinperimetrically encases the bundles of cellulose fibers, but does notfill the familiar canal within them. Note the annuli with central blackspots indicated by the arrows.

For more detailed scrutiny of one of the annuli, as well as the microstructure surrounding it, reference may be had to the micrograph of FIG.11, which is an enlargement of the area about the annulus indicated bythe longer-tailed arrow on micrograph II, which show not only the opencanal in, but a resin deposit on the upper side thereof, as well as thesheath of resin encasing, the bundle of fibers seen in cross-section. Italso shows numerous other bundles of fibers (some cracked, probably fromthe sectioning operation) with prominent highlights indicative of theresinous sheath about the fiber bundles, and also the bonding together,by the resin, of random oriented fiber bundles.

While it has not yet been ascertained with certainty what particularchemical or physical phenomenon causes the phenolic resin to have apreferential affinity for attaching itself to the exterior increments ofthe fiber bundles rather than filling the canal within the bundles,several possible theories present themselves, to wit: the resin moleculeis too large to enter the canal; the cellulose fibers are more readilywet by the water-catalyst solution (in the surface-treating mixture)than by the resin, with the result that the water (with some of thecatalyst dissolved therein) is quickly absorbed by the fiber bundles,thereby deserting the larger molecules of resin which are more or lessfiltered out and deposited as a sheath about the perimetrical incrementsof fiber bundles where the uncured resin in the sheath is catalyzed fromthe inside, out, as well as from the outside, in. Regardless of which,if any, of the theories may prove correct, the fact that a sheath ofresin circumscribes the fiber bundles, substantially adds to thestrength of those bundles, and, together with the cross bonds formed bythe resin between different bundles of fibers, clearly provides strengthsufficient to enable a paperboard box to maintain its shape andsubstantial strength after the cellulose fibers have become limp fromprolonged presence in water. Moreover, the degree of brittleness whichhas heretofore militated against the success of resin-treated paperboardin containers (hopefully intended for use under wet conditions) isreduced to insignificance by the fact that, while the resinous sheathabout the fiber bundles (as well as the peripheral increments of suchbundles which are penetrated by the resin) may fracture during scoringand folding (which is where the objectionable brittleness has heretoforemanifested itself), most of the cellulose fibers are not penetrated bythe resin, and hence remain sufficiently flexible to hold the fiberlattice intact under scoring and folding conditions.

As typifying the Wet/Dry Strength Retention Factors obtainable withRegular Slotted Containers (10 × 12 × 10) and Half Slotted Containers(16 × 12 × 12) made of corrugated paperboard treated in accordance withthis invention, in contrast with board not, or only partially, sotreated, the following test results are significant:

                  TABLE II                                                        ______________________________________                                        TABLE I                       After    Wet/                                   Components                                                                             Component  Dry       24 Hours Dry                                    Combination                                                                            T.A.I.     (Tappi 402)                                                                             Under Water                                                                            S.R.F.                                 ______________________________________                                        4-7-4    None       645#      Zero                                            4-7-4    Medium only                                                                              675#      150#     22%                                    4-7-4    Liners &   817#      274#     33%                                             Medium                                                               3-6-3    Liners &   1344#     460#     34%                                             Medium                                                               1-5-1    Liners &   2688#     805#     30%                                             Medium                                                               ______________________________________                                    

In Table II, and the Tables to follow: "T.A.I." means treated accordingto this invention, i.e., liners surface treated with phenolic resin onthe concealed side only, medium so surface treated on both sides,combined (by the above-described resorcinol-starch adhesive), and cured;"S.R.F" means Strength Retention Factor; and "#" means pounds.

As typifying the results obtainable from corrugated container-board, insome of which the paperboard components were, and some not treated inaccordance with the present invention, the test results of Tables IIIand IV are cited:

                  TABLE III                                                       ______________________________________                                        FLAT CRUSH (Tappi 808-OS-65) OF                                               CORRUGATED CONTAINER-BOARD                                                    TABLE I                       After    Wet/                                   Components                                                                             Components Dry       24 Hours Dry                                    Combination                                                                            T.A.I.     (Tappi 402)                                                                             Under Water)                                                                           S.R.F                                  ______________________________________                                        2-5-2    None       45 p.s.i. Zero                                            2-5-2    Liners &   63 p.s.i. 34 p.s.i.                                                                              54%                                             Medium                                                               1-5-1    None       74 p.s.i. Zero                                            1-5-1    Liners &   72 p.s.i. 32 p.s.i.                                                                              44%                                             Medium                                                               2-6-2    Liners &   29 p.s.i.  9 p.s.i.                                                                              31%                                             Medium                                                               3-6-3    Liners &   29 p.s.i.  9 p.s.i.                                                                              31%                                             Medium                                                               ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        COLUMN CRUSH (Article Entitled "Compression                                   Strength Formula For Corrugated Board"                                        August 1963 issue of "Paperboard Packaging"                                   Published by Board Products Publishing Co.,                                   Chicago) OF CORRUGATED CONTAINER-BOARD                                        TABLE I                       After    Wet/                                   Components                                                                             Components Dry       24 Hours Dry                                    Combination                                                                            T.A.I.     (Tappi 402)                                                                             Under Water                                                                            S.R.F.                                 ______________________________________                                        2-5-2    None       68 p.i.w. Zero                                            2-5-2    Liners &   71 p.i.w. 28 p.i.w.                                                                              39%                                             Medium                                                               1-5-1    None       74 p.i.w.  6 p.i.w.                                                                               8%                                    1-5-1    Liners &   81 p.i.w. 25 p.i.w.                                                                              31%                                             Medium                                                               ______________________________________                                    

In Table IV, "p.i.w." means pounds per inch of width of the specimentested. Neither embrittlement of objectionable degree nor delaminationoccurred with specimens subjected to the tests reported in Tables II,III or IV.

Wet/Dry Strength Retention Factors as high as 30% have not, to ourknowledge, been heretofore attainable with the consistency required forindustrial production, or without objectionable embrittlement ordelamination, from combined corrugated board (or boxes made therefrom)which has been under water for 24 hours. The Wet/Dry Strength RetentionFactor suffers severe diminution upon delamination of the combinedboard, and hence that Factor is to be construed herein as implying thatlamination is maintained throughout the period of under-waterpreconditioning for, and during, tests. However, the Strength RetentionFactor only partially relates the significance of the rigid-when-wetcharacteristics achieved by this invention. A further importantcharacteristic of container-board made in accordance with thisinvention, and of containers fabricated therefrom, is the longevity ofthe rigid-when-wet strength without delamination as illustrated by thegraphs in FIG. 8, which depict, comparatively, the change in Wet/DryStrength Retention Factor over much longer periods of under-waterpre-conditioning for test. In the comparative tests, whose results arereflected in FIG. 8, three series of corrugated paperboard containerswere involved, to wit: (i) fabricated from corrugated board whosepaperboard components (1-5-1) were surface-treated, combined and curedin accordance with this invention, and whose characteristics are plottedby the solid black line; (ii) fabricated from standard commercialcorrugated container-board, whose Wet Strength paperboard components(1-5-1) were the same as those in (i), but without treatment inaccordance with the present invention, and whose characteristics areplotted by the circular dots in FIG. 8; and (iii) fabricated as in (ii),but subsequently impregnated with wax in the manner which, insofar as weare informed, has theretofore been considered to be the mostindustrially acceptable way to produce boxes having characteristicswhich approach those of boxes made in accordance with the presentinvention.

It will be apparent from FIG. 8 that the Wet/Dry Strength RetentionFactor of boxes made per (ii) above bottoms out to zero afterapproximately 15 hours under water; that the Wet/Dry Strength RetentionFactor of boxes made per (iii) above declines to 30% at 23 hours, andbottoms out to zero after approximately 48 hours, under water; but incontrast, boxes made in accordance with this invention maintain theirWet/Dry Strength Retention Factor well above 30% after more than 90hours under water, e.g., 35% at 96 hours without delamination. Theunderwater period required for boxes made in accordance with the presentinvention to bottom out at zero Wet/Dry Strength Retention Factor or todelaminate has not yet been ascertained, but in any event, boxes whichmaintain a Wet/Dry Strength Factor of at least 30% for a period of 24-30hours under water without delamination are usually adequate for theprimary purpose of the invention, namely, the intranational shipping offresh comestibles from source to market under wet conditions.

While the invention has been described with particular reference tocorrugated paperboard containers, it is also applicable to other typesof laminated paperboard, such as the so-called "solid fiber"container-board, in the manufacture of which three or more plies ofpaperboard, comparable with the liners and (uncorrugated) medium ofcorrugated container-board are laminated together, and can besurface-treated, as hereinbefore described, with phenolic resin on theconcealed side of each outside ply, and/or both sides of at least one ofthe intervening ply or plies.

From the foregoing description, those skilled in the art will readilyunderstand that the invention achieves its objects, and providespaperboard components for combination with each other to producelaminated paperboard, and containers made therefrom, which are readilywettable and substantially water-absorptive, but maintain a sufficientpercentage of their dry strength to enable them to be packed, handled,and shipped for long distances, or stored for long periods of time,under wet conditions, while laden and stacked one upon the other.

Having thus described the invention, what is claimed and desired to besecured by Letters Patent is:
 1. Rigid-when-wet laminated paperboardconsisting of at least three plies, each having two faces,aromatic-alcohol-containing adhesive means securing said plies togetherto provide two exposed faces and at least four concealed faces, two ofsaid concealed faces forming a corrugated paperboard web, said exposedfaces being water absorbent, and at least two non-contiguous ones ofsaid concealed faces being impregnated through roller application with aprinted film of a controlled amount of a phenolic resin contained withina 35 to 65 percent by weight solution of a vaporizable liquid vehicle,said plies containing multitudinous bundles of cellulose fiberssurrounding a central canal, the amount of resin solution being between3.3 to 3.9 pounds per thousand square feet per face of each ply, saidresin forming a sheath about and penetrating the outer perimetricalincrements of said bundles while leaving their central canalssubstantially devoid of resin, wherein the laminated paperboard remainsabsorbent of moisture but retains structural integrity due to thepresence of the roller applied phenolic resin to particular of itsfaces.